Insertion of transgenes into the mouse genome occasionally results in disruption of important endogenous genes. When bred to transgene homozygosity, such transgenic mice may exhibit mutant phenotypes resulting from improper functioning of the endogenous gene. We have encountered two different insertional mutants exhibiting developmental abnormalities. One line of mice (line 2627) harbors approximately 15 copies of a transgene on chromosome 11. The phenotype of this line is characterized by hemivertebrae, vertebrogenic spina bifida occculta, fusions of vertebral bodies, and short, kinky tails. This phenotype is reminiscent of undulated, a mutation in the paired-box of the Paxl protein encoded by the paxl gene on chromosome 2. Since Paxl and the gene with the transgene insertion map to different chromosomes, yet produce similar phenotypes when mutated, it is possible that the two gene products act along the same metabolic pathway. To clone the endogenous gene on chromosome 11, we have prepared genomic libraries in lambda phage and are currently screening them for the presence of DNA sequences flanking the transgene. A second line of transgenic mice (VGA 9) harbors approximately 50 copies of a transgene on chromosome 6. Homozygous transgenics show a complete loss of skin, complete or near-complete loss of eve pigmentation, microphthalmia, and a severe hearing deficiency of at least 50 dB as analyzed by brain stem auditory evoked potential measurements done in collaboration with Dr. Kenneth Grundfast from the LMO, NIDCD. The hearing deficiency may be the consequence of an underdevelopment of the stria vascularis of the cochlea where the intermediate cell layer with its melanin-producing cells is missing. Thus, the insertion affects, among other cell lineages, the proper development of the neural crest-- derived pigment cells. It is allelic with, and phenotypically similar to, the mutation mi (not cloned), and shares features with two other well characterized mutations, W (a mutation in the receptor kinase c-kit), and S/ (a mutation in the growth factor steel). The overlap of phenotypes suggests that these three mutations and the VGA 9 insertion affect genes of the same metabolic pathway, Of particular interest is the connection between VGA 9 and the human Waardenburg syndrome, a syndrome characterized by hereditary deafness and pigment abnormalities. Identifying the gene affected in the transgenics may provide molecular probes useful to characterize patients with this syndrome, and may help to understand the pathogenesis of this disease.